Radiocommunications Agency  EMC Awareness

Power harmonic reduction

What this technique is used for

Many power switching systems – variable speed motor drives, uninterruptible power supplies, inverters and other high-power power conversion applications – use both low and high-frequency switching techniques to transform power from their input to output with the maximum efficiency. These switching waveforms are rich in harmonic content and couple readily to other victim systems, particularly audio and LF/MF radio equipment. To prevent this, the harmonic content has to be controlled.

How this technique is used

The first technique, which should be used at the earliest design stage is to reduce the dV/dt and dI/dt (the rates of change of voltage and current with time) associated with the semiconductor switching in the AC-DC converter circuits.

The second technique involves filtering the a.c. power supply input to the power converter.

How power harmonic problems arise

Harmonics can be caused either by switching the a.c. power supply frequency directly, or by converting it to DC and then switching the resultant power at a higher frequency. Depending on the speed of switching of the semiconductors in the power converter, their harmonics can extend up to several kHz in the low-frequency case or to several MHz if high-speed, efficient devices are used. These harmonics fall in the audio frequency band, and so interfere with telephone audio circuits, or can affect radio reception in the range above 100kHz.

Mains distribution transformers into the medium-voltage (MV) and high-voltage (HV) distribution networks do not block these emissions. These couple to other equipment and systems, via their connected cables, by…

• Direct conduction via the a.c. power distribution network

• Earth potential differences caused by the power supply network or by common-mode return currents

• Induction or common-mode coupling from the power distribution cables.

As well as coupling back through the supply, cables attached to the output of the power system – such as from a drive to a motor – can also couple the interference out by similar mechanisms. Installation of such systems must use a combination of filtering and cable segregation techniques.

Key issues in employing this technique

Switching time, efficiency and cost

Everything about high-power converters is costly, and they handle so much power that a few percent difference in efficiency can make a big difference to their economic justification and costs of operation, as well as the costs of their cooling systems.

The lowest cooling system costs and highest efficiency usually requires the fastest possible switching times, leading to high values for dV/dt and dI/dt and correspondingly high levels of energy in higher-frequency harmonics and more chance of creating telephone and radio interference.

Filtering is the only remaining technique, and at the high powers involved the filters can be comparable in size (and cost) with the power converter itself. Filtering lower frequencies also requires significantly larger filter components. Filtering harmonic noise from the power input can mean filtering down to its 5th harmonic (250Hz for a 50Hz mains powered system, 83.33Hz for a 16.67Hz traction power system). It can make more economic sense to reduce the dV/dt and dI/dt and pay more for cooling systems and lost efficiency – but some designers and their managers may want to go for what appears to be the lowest-cost most efficient design, ignoring EMC considerations until later in the project when costly filters have to be added because it is too late to alter the fundamental design.

Reducing dV/dt and dI/dt

This can require using rectifiers (or ‘active rectifiers’) with slower switching times, lower efficiencies and higher thermal losses. Series inductors can be used instead or as well, to reduce the levels of harmonic currents and so reduce the harmonic emissions into the mains supply.

Notch filtering and line reactors

Ordinary low-pass EMC filters with corner frequencies low enough to give attenuation at such low frequencies would be very large, heavy and costly. So resonant filter designs are mostly used instead, with notch characteristics. Notch filters are designed to resonate at a particular harmonic in such a way as to attenuate it strongly. Several of them may be required, each tuned to a different frequency.

Another common way to improve the harmonic performance of a power converter is to include a large inductor, known in this industry as a “line reactor”, in series with the power supply input. The impedance of this component rises with frequency and reduces the amplitude of the higher order harmonics, though it is not so effective as a tuned notch filter.

Six-phase converters

High-power converters don’t use single-phase supplies. Three-phase rectifiers with a balanced load on each phase do not generate triplen harmonics (3rd, 9th, 15th, etc) at their power inputs.

It is possible to use an input transformer with both star and delta secondaries to achieve a six-phase supply, and a six-phase rectifier has no 5th or 7th harmonics either (if the load is equally balanced on all of the six phases). So a six-phase converter (sometimes called a twelve pulse converter) should not emit high levels of harmonics into it’s a.c. power supply at frequencies less than its 11th harmonic.

The use of six-phase converters considerably eases the cost and size of the filters that would otherwise be needed in the a.c. power supply.